One quick and dirty tool that I like to use to monitor I/O performance at the file level is Resource Monitor (in Windows Server 2008 or 2008 R2). As you see below, the Disk portion of the display lets you quickly look at reads, writes, total B/sec, and response time for individual files. The columns are sortable, so it is very easy to get a better understanding of your I/O patterns and possible bottlenecks.

In the example below, you can see that the R: drive is a little less happy than the other drives on this server. This can be useful information to know when you are trying to determine where to locate data and log files on a server. One thing to be aware of is that Resource Monitor can be fairly expensive to run (I often see it using 5-7% CPU on a 16-core server), so you should not just leave it running all of the time.